I have no Idea where to start

I'd probably start with the floor pans. These cars are uni-body, so having a fully intact floor is important. And like others have already mentioned, once you have the carpet out you can see what you're really dealing with. This is what mine looked like when I removed the carpet:

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Auto Body Specialties sells pans that are just the foot well area if that's all you need. They're nice quality and fit really well. Almost done with the first side:

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And as you may have already learned, the foot well areas typically rust from the inside out, so replace the wiper pivot seals if they haven't already been done
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